The resistance of a polyester to hydrolysis greatly depends on the number of carboxyl end groups per unit weight. It is known that an improvement in resistance to hydrolysis can be achieved by closing off these carboxyl end groups by chemical reactions. Reactions with aliphatic, aromatic and also cycloaliphatic mono-, bis- or polycarbodiimides have already been described in several incidences as such "closing-off" of the carboxyl end groups.
Thus, for example, German Offenlegungsschrift 1,770,495 describes stabilized polyethylene glycol terephthalates which have been obtained by the addition of polycarbodiimides. Because of the slower rate of reaction which is generally to be observed with polycarbodiimides, it is necessary to endure a relatively long residence time of the polycarbodiimide in the polyester melt. For this reason, polycarbodiimides have already been added during the polycondensation reaction of the polyesters. However, a number of disadvantages are associated with such a procedure. For example, a large number of by-products are formed because of the long residence time, and where relevant, the actual polycondensation reaction of the polyester is also impeded.
Japanese Published Specification 1-15604/89 requires the use of an excess of 0.005 to 1.5% by weight of the monocarbodiimide specifically recommended. The Japanese Published Specification furthermore states that the use of polycarbodiimides does not correspond to the prior art already achieved.
Japanese Laid Open Document P 05163612 A and JP 05627639 B disclose the use of monomeric 0.5-0.25 weight % of N,N'-bis(2,6-diisopropylphenyl)carbodiimide as preferred to stabilize polyesters of poly(ethylene naphthalate). However, in section 0017! of Japanese Laid Open Document P 05163612 A, it is clearly stated that "Carbodiimide compounds with multiple functional groups such as biscarbodiimides are not preferable, since they cause rapid, high-degree polymerization of poly(ethylene naphthalate) so that it will be difficult to knead together with poly(ethylene naphthalate)."
Disclosure of the use of monomeric 0.5-0.25 wt % of N,N'-bis(2,6-diisopropylphenyl)carbodiimide to stabilize polyesters of poly(ethylene 2,6-napthalenedicarboxylate) is found in JP 05163612 A and JP 05627639 B to Teijin LTD.
U.S. Pat. No. 5,246,992 discloses the use of a combination of mono- and/or bis- carbodiimides with polycarbodiimides as hydrolytic stabilizers for polyesters. The main constituent of the polyesters disclosed is terephthalic acid, and other, preferably para or trans compounds, such as for example, 2,6-naphthalenedicarboxylic acid, or else p-hydroxybenzoic acid. Example 3 of this patent discloses a comparative example of the use of a polycarbodiimide (15% of the polycarbodiimide in polyethylene terephthalate) alone with the polymer material. Reduced thermal and hydrolytic resistance compared to the prior art was observed in this example. Also, col. 6, lines 49-51 of U.S. Pat. No. 5,246,992, states that the sole use of polycarbodiimides did not lead to the required stabilization.
U.S. Pat. No. 4,829,681 discloses a paper machine cloth comprising interwoven machine direction and cross-machine direction yarns prepared from terephthalic acid and ethylene glycol with improved resistance to hydrolysis.
U.S. Pat. No. 5,169,499 also discloses a paper machine cloth comprising interwoven machine direction and cross-machine direction polyester yarns based on polyesters prepared from terephthalic acid, isophthalic acid, and 1,4-cyclohexanedimethanol with improved resistance to hydrolysis. The polyesters disclosed in this invention are stabilized by commercial polymeric carbodiimides manufactured by Rhein Chemie Corporation under the tradenames Staboxal.TM. P-100. Rhein Chemie Corporation (Germany) manufactures a variety of carbodiimide products which are used to improve the resistance of polyester products to hydrolysis.
None of the above described references fully address the problem of lack of hydrolytic stability in PEN compositions.
Therefore, there is a need in the art for PEN compositions with improved hydrolytic stability.